The present invention relates to an antenna device for a mobile communication terminal, and more particularly to an antenna device for a mobile communication terminal which is suitable for a mobile phone.
There are various types of antenna devices for a mobile phone. In particular, recently, a combination of a pull-out type rod-shaped (whip) antenna and a built-in antenna has been proposed. This has an object to satisfy a requirement for such an antenna structure as to sufficiently maintain an antenna sensitivity and to have no protrusion during the storage of an antenna in respect of a design.
As a result of the use of the two antennas, there has been proposed a structure in which a feeding circuit is connected to the rod-shaped antenna or the built-in antenna through a changeover switch.
FIGS. 14A and 14B are diagrams showing a switching circuit having a structure using the two antennas. In FIG. 14A, a rod-shaped antenna WA is pulled out and is connected to a feeding circuit FC through an impedance matching circuit MC1. On the other hand, in a built-in antenna IA, a switch member SB1 disconnects a feeding terminal IFC1 from a matching circuit terminal MFC1 and a switch member SB2 disconnects both ground terminals IGC1 and GC1, and only the rod-shaped antenna WA is connected to the feeding circuit FC.
In contrast, in FIG. 14B, the rod-shaped antenna WA is disconnected from the matching circuit MC1 and is grounded through a matching circuit MC2 having a high impedance. The feeding terminal of the built-in antenna IA is connected to the feeding circuit FC through the switch member SB1, and furthermore, the ground terminal of the built-in antenna IA is connected through the switch member SB2.
In FIGS. 15A and 15B, in order to change over the rod-shaped antenna WA and the built-in antenna IA, a cam CM for pulling out the rod-shaped antenna WA and operating in response to a storage operation is provided and the button of a switch SW is caused to appear frequently by means of the cam CM.
Thus, the rod-shaped antenna WA and the built-in antenna IA can be changed over.
In a mobile phone, a frequency band to be used has further been expanded and a multiband antenna has been required.
For the multiband communication, however, the matching circuit MC2 in FIGS. 14A and 14B is to carry out impedance matching for at least two frequency bands, which is difficult. For this reason, it is hard to constitute the multiband antenna in the antenna having the above structure.
It is therefore an object of the invention to provide an antenna device for a mobile communication terminal which changes over and uses a band-shaped antenna and a built-in antenna and does not cause impedance mismatching for multiband communication.
In order to achieve the above object, according to the invention, there is provided an antenna device, incorporated in a mobile communication terminal, comprising:
a first antenna, movable between a first position pulled out from the communication terminal and a second position accommodated in the communication terminal;
a second antenna, built in the communication terminal;
a power feeding circuit;
an impedance matching circuit;
a first switch, which electrically connects only the first antenna with the power feeding circuit via the impedance matching circuit when the first antenna is placed at the first position, and electrically connects only the second antenna with the power feeding circuit when the first antenna is placed at the second position; and
a second switch, which grounds the second antenna in cooperation with the first switch, when the first antenna is placed at the second position.
In such a configuration, the grounding circuit of the second antenna is disconnected by the second switch cooperated with the first switch when the first antenna is stored in the communication terminal. Therefore, when the feeding circuit is connected to the second antenna, an element related to the first antenna is isolated from the feeding circuit. Thus, the feeding circuit is not influenced by the element related to the first antenna. As a result, it is possible to provide an antenna which does not cause a drawback by multibanding.